Detachable pole assembly for dynamo electric machine stator

ABSTRACT

A dynamoelectric machine including stator and armature structures. The stator includes a yoke and a plurality of pole shoes carried thereby which include slots for receiving compensating coils. Strand-like elements are tightly packed within the slots between the compensating coils and the slot walls and are embedded within a hardened binder material, to support the coils against oscillation. Each pole shoe includes tip and base sections defining a pole face. The tip section comprises plurality of lamination plates which have aligned apertures defining an opening extending through the tip section remote from the pole face. A connecting bar is disposed within this opening and has fastener elements mounted on each end thereof to clamp the lamination plates together. Threaded bolts are received in the connecting bar to draw the lamination plates toward the base section in uniform fashion. The yoke comprises a plurality of lamination plates which are arranged in mutually abutting relationship. A plurality of attachment flanges are welded across the outer edges of yoke plates to couple the plates together. The shoe base section is detachably secured to the yoke such that shoulder portions of the base section serve to clamp primary current windings in place against the yoke.

Jan. 7, 1975 yoke and a pluy which include slots r receiving compensating coils. Strand-like elements p and p section pening extending m the pole face. A

connecting bar is disposed within this opening and has fa end thereof to plates g relationship.

[5 7] ABSTRACT DYNAMO ELECTRIC MACHINE STATOR A dynamoelectric machine including stator and arma- Walter F. Boesel, Palos Verd s ture structures. The stator includes a Estates, Calif. rality of pole shoes carried thereb fo g g Corporation Los Angeles are tightly packed within the slots between the compensating coils and the slot walls and are embedded July 27, 1973 within a hardened binder material, to support the coils against oscillation. Each pole shoe includes ti base sections defining a pole face. The ti comprises plurality of lamination plates which have U.S. 310/218, 310/214, 310/224, aligned apertures defining an 0 310/194 through the tip section remote fro H02k 1/18 310/217, 218, 224, 214, stener elements mounted on each 310/183-187, 194, 192, 210, 254 clamp the lamination plates together. Threaded bolts are received in the connecting bar to draw the lamina- References Cited tion plates toward the base section in uniform fashion. UNITED STATES PATENTS The yoke comprises a plurality of lamination which are arranged in mutually abuttin A plurality of attachment flan United States Patent 1 1 Boesel DETACHABLE POLE ASSEMBLY FOR [75] Inventor:

[73] Assignee:

[22] Filed:

[21] Appl. No.: 383,174

[51] Int. Cl.

[58] Field of Search S e S CC m wk m eP n ai 5 S M m 6 t w m w s a n s 0 a m 0 P 6 Claims, 6 Drawing Figures the outer edges of yoke plates to couple the plates together. The shoe base section is detachably secured to the yoke such that shoulder tion serve to clamp primary current windin against the yoke.

0 7d EVA 8444 87 4 l l 22 2 l 1 l 111:1. nnun mm n J "b OOOOHOOMO H a) A 1111 11 1 e 333303303 b u mmuu A Q mumuuunnu u" n n u "n t 8 uu" d n n e n u t m m m l uumm M- m u m u m m t mm H mm uE m m n m n ma 3 m m 61 "VJ" .msamkfle m mwmmmnme k .H OaO aO mm... FDTLKPHRW Mhfl fi m 3524479 r M 55666 H0 999999999 n ,K HHHHHHH m UNA 2595244 m E .1 0 m 8 257278 mOo o720/o65 YQ wnr 2930009937 r 2653 75 3 w n a .2333 PAM DETACHABLE POLE ASSEMBLY FOR DYNAMO ELECTRIC MACHINE STATOR BACKGROUND AND OBJECTS OF THE INVENTIONS This invention relates to dynamoelectric machines such as electric motors and generators.

In dynamoelectric machines having a commutating armature there is always present the problem of a cross-magnetizing field in the armature created by the load-current therein. This problem is of little consequence in smaller machines, but when the machine size becomes greater than a few horsepower, and when maximum overload and broadest speed range are desired, it is advantageous to provide the machine with compensating windings coupled in series with the armature. This tends to neutralize the cross-magnetization to the extent possible.

Problems occur in attempting to adapt a dynamoelectric machine to receive the compensating coils. While it is desirable to provide an effective orientation of the compensating coils, it is also important not to impair overall machine performance or unduly complicate the assemblageand disassemblage of the machine components.

It has been previously proposed to provide dynamoelectric machines with detachable pole shoes which contain slots for receiving the compensating coils. Exemplary of such a proposal are Rimkus U.S. Pat. No. 2,745,031, issued May 8, 1956 and Potter U.S. Pat. No. 2,679,607 issued May 25, 1954.

In Rimkus, the coil-receiving slots are intended to be open-ended and closed-sided. One assemblage problem posed by closed-sided slots is the difficulty involved in threading small-stranded conductors axially therethrough, an operation akin to attempting to push a rope from the end. In addition, Rimkus proposes that a groove be formed in the pole face for the reception of a cross-bar intended to serve in mounting the pole shoe to the yoke. Such a groove constitutes an interruption in the pole face and might tend to reduce the fluxconducting efficiency of the pole shoe.

The Potter patent proposes the use of a pole shoe having a main pole section and removable teeth which cooperate with the main pole section to define coilreceiving slots. The teeth are arranged to abut and support the coils. Inconveniences may arise in that removal of the excitation windings would require removal of the compensating windings, and vice-versa, in an arrangement where one set of mounting bolts functions to both secure the shoe sections together and attach the shoe to the yoke.

One serious problem associated with the presence of coils within pole slots concerns a tendency of the coils to oscillate, or vibrate, relative to the slot walls during machine operation. This behavior, besides being noisy, produces considerable wear on the coils and necessitates the use of relative thick insulation around the coils. Use of such insulation reduces the number of wires which can be placed in each slot and thereby places an unreasonable limitation on the compensating effect of the coil by limiting the number of ampereturns. Attempts have been made to eliminate this oscillation, as by inserting a non-magnetic, non-rigid compound between the slot walls and the coil. The effectiveness of this approach has been found to be of only temporary duration, however. Moreover, ifa compound is used in slots which are slightly open at the pole face, parts of the compound could become detached, leaving depressions in the pole face and resulting in the introduction of loose debris within the machine.

Construction of the yoke portion of a dynamoelectric machine yoke may be conventionally accomplished by brazing together a series of laminated plates. This arrangement can be disadvantageous in that the brazing alloy typically occupies space which could otherwise be occupied by yet more plates, the addition of which could cut hysterisis losses and increase machine efficiency.

It is, therefore, a general object of the invention to provide novel methods and apparatus for minimizing or obviating problems of the sort previously discussed.

It is another general object of the invention 'to provide novel methods and apparatus for optimizing the performance of dynamoelectric machines.

It is a particular object of the invention to provide novel methods and apparatus for effectively resisting oscillation of compensating coils within dynamoelectric machine slots.

It is another object of the invention to provide a novel dynamoelectric machine characterized by simplified assemblage and disassemblage.

It is yet another object of the invention to provide a novel dynamoelectric machine which exhibits minimal hysterisis losses and maximum efficiency.

BRIEF SUMMARY OF THE PREFERRED EMBODIMENTS OF THE INVENTION In accomplishing at least some of the foregoing objects, the present invention entails a dynamoelectric machine which includes stator and armature structures, a plurality of primary current windings, and a plurality of pole shoes carried preferably by the stator structure.

In one feature of the preferred embodiment, each pole shoe comprises base and tip sectionswhich are detachably secured to each other and define slots for the reception of compensating coils. A plurality of strandlike elements are disposed within each slot and are tightly packed between the compensating coil and wall portions of the slot. A binder material, such as a resin for example, is poured into each strand-filled slot and allowed to harden so as to embed the strand-like elements therein. In this fashion, the resin-impregnated, strand-like elements function to restrain the compensating coils from oscillating within the slots.

In another feature of the preferred embodiment, the tip section of each pole shoe comprises a plurality of lamination plates. The plates include aligned apertures which define an opening extending through the tip section remote from a pole face that is defined by the outer edges of the tip plates. A connecting bar is disposed in the opening and has fasteners mounted on the ends thereof to firmly clamp the tip plates together. Threaded bolts extend between the base and tip sections and are received within the connecting bar to draw together the base and tip sections. The connecting bar serves to draw the tip plates toward the base section in a generally uniform manner thereby preventing the occurrence of relative displacement of the tip plates. The fasteners at the ends of the connecting bar may comprise anchor brackets which are attached to the stator to assist in mounting the tip section to a yoke portion of the stator.

the yoke plates define a plurality of grooves around the yoke. These grooves receive attachment flanges which are welded to the outer edges of the yoke plates. In this manner, a maximum number of yoke plates per area may be provided.

The base section of each pole shoe may be detachcross-flanges 30. These flanges 30 extend along the outer yoke periphery within grooves 32 formed by aligned recesses in the yoke plates 24. The flanges 30 ably secured to the yoke by means of threaded connectors. Shoulder portions of the base section may be utilized to clamp the primary current windings to the yoke when the pole shoe is attached to the yoke by the threaded connectors. With such an arrangement, the pole shoe may be removed from the yoke to allow replacement of the current windings, without necessitating undue disturbance of the compensating coils.

THE DRAWINGS in FIG. 2;

FIG. 4 is a perspective view of a connecting bar element according to the invention;

FIG. 5 is an enlarged cross-sectional view taken along the line 55 in FIG. 3, but not necessarily being to relative scale; and

FIG. 6 is a cross-sectional view of a modified form of the machine in which supplemental connecting apparatus is to be employed in accordance with the invention.

DETAILED DESCRIPTION OF THE 'PREFERRED EMBODIMENT OF THE INVENTION A dynamoelectric machine 8 incorporating preferred forms of the present inventive features includes a stator field structure 10 enclosing a rotor armature structure 12. The armature 12 is journaled for rotation within the stator 10 and may be constructed in accordance with known principles. V

The stator 10 includes a yoke 14 which. carries a plurality of spaced salient pole shoes 16 disposed adjacent the periphery of the armature 12. Magnetic flux which is produced during machine operation is conducted between the stator and armature by the pole shoes 16. A system of primary electrical windings 15 is mounted on the stator to conduct current in the usual manner.

Each pole shoe 16 includes a base section 18 and a tip section 20. The shoe sections 18 and 20 are relatively separably connectable, and the base section 18 is separably connectable to the yoke, by means to be subsequently described.

The yoke 14, the shoe base section 18, and the. shoe tip section 20 are each of laminated construction, being respectively defined by a plurality of laminationplates 24, 26, and 28, as may be viewed in FIG. 3. The plates 24 forming the yoke are held together by a plurality of may be welded to the yoke plates 24 to hold the plates in side-by-side abutting relationship. I I

The plates 26 forming the base section 18 are clamped together in side-by-side abutting relation by means of a plurality of clamping bolts. 34, depicted in FIG. 1.

The manner in which the plates 28 of the tip section 20 are mounted constitutes a significant aspect of the invention and will be discussed subsequently in detail.

The base section 18 includes a pair of shoulder portions 36. These shoulders 36 serve to clamp the windings 15 to the yoke 14 when the base section is attached to the yoke by bolts 37. Aligned openings 38, 40, and 42 in the cross flanges 30, the yoke 14, and the base section 18 receive the bolts 37. The openings 42 are threaded to receive threaded ends of the bolts 37. The mounting end 44 of the base section 18 seats within a groove 46 of the yoke 14 to maintain the pole shoe in proper orientation.

If desired, the windings 15 may be wrapped around spools (not, shown), with each spool being inserted onto a shoe base section.

The laminated plates 28 which form the tip section .20 are arranged in mutually abutting, side-by-side relationship. Aligned apertures in theplates 28 define an opening 48 extending through the tip section. Situated within this opening 48 is a connecting bar 50 which has threaded studs 52 projecting from opposite sides of the shoe tip 20. Fastener elements, such as washer and nut assemblies 54 for-example, are coupled at the ends of the connecting bar to hold the plates 28 tightly together as a unit.

The connecting bar 50 includes a plurality of threaded holes 56 which are arranged to threadedly receive associated bolts 53 for securing the shoe sections 18 and 20 together. The shank and head sections of these bolts are respectively located within passages 60 and 62formed in the base section 18. The threaded terminal ends of the bolts 58 are received within bores 64 formed in the shoe tip 20.

It should be understood that when the connecting bar 50 is drawn toward the base section 18 by the bolts 58, it will engage all of the tip plates 28 and draw them toward the base section 18 in a uniform manner. In this fashion, there will be no relativev displacement of the tip plates 28.

Moreover, the connecting bar 50 is disposed remote from thepole face 70, defined by the outer edges of the plates 26 and 28, to avoid distorting the generally smooth configuration thereof.

It will be apparent that with the connecting bar 50, together with the fasteners 54, serving as a means for pressing the plates 28 together, the need for rivets or other coupling devices which might increase the size of the shoe tip will be eliminated.

In the event that relatively short pole shoes are employed, the mounting of the pole shoe may be accomplished by bracket assemblies, one of which being depicted inFIG. 6. The bracket assemblies are disposed at opposite ends of the connecting bar 50'. An anchor bracket 66 is mounted at each side of the shoe tip 20 and includes an aperture 67 which receives a threaded stud 52 of the connecting bar 50'. The nut and washer assembly 54' holds the anchor bracket 66 tightly The strands 112 are fabricated of a rigid non metallic' material such ,as glass, high temperature nylon, or as:

against the shoe sections 18 and20. The bracket 66 illeludes an outwardlyextending leg 68. A' fastener, such as a bolt 71, for example, passes through-this leg to hold 3 the bracket 66 to the yoke 14, and .thu'saugment the 1 connection between the'pole shoeand the yoke and the connection betweenthe shoesections 18 and 20.

The base section 18 includes ajwall 76 that defines a ca'vity withinwhich thetipjsection 20 is normally situated. The outer-most edg e's of the tip section 20'a'nd'the base section 18, when aligned, fo rm the curved pole face 70. When these shoe sections18 and 20 are coupled together, wall portions of these section's'cooperate to form a plurality of slots 80, 82,84, 86'for the reception of compensating coils'. The front and intermediate sides of the slots 82 are defined by walls 88, 90, 92 on the shoe tip 20, 'while the back sides of these slots 82 6 sary to properly orient-the shoe sections in establishing a'substantiall-y. smooth pole face 70.

Once a condition is established in which the strands 112 are tightly packed between the compensating coils and the slot walls, and the pole face-defining edges of the'shoe sections are appropriately aligned, the shoe may be laid on-end to receive liquidified, i.e., pourable,

resin 114, such as polyesteror silicone, which is'poured downwardly into the strand-containing recesses. By

capillary action, the strands 112 conduct the resin downwardly therebetween. During hardening, the resin 1 becomes bonded to the embedded strands, the coil and and 84 are defined by the eavity-defining wall 76 of the if shoe base 18.

The slots 80 and 86 each have a bac kjside, one intermediate side, and a front side portion formed by the cavitydefining wall 76 of the base "18' (FIG: 2). A wall I 100 of the shoe tip 20 cooperates'with the wall 76 to form the other intermediate side and the remainder of the front side of'each slot80 and 86. A small ga'ap 102 is created at the front of slots 80 and 861between the walls 76 and 100. This gap 102 may be onthe'order of cles into the machinery. 1

one-sixteenth or one-thirty-second of an inch wide, for

- x lef Aswill beapparent, with the shoe sections 18 and in a detachedcondition, the slots 80,82, 84', 86'will be open to facilitate convenient insertionof compensating coils 104, 105, 106, l07.therewithin. Referencermaybe had to the inven' tors co-pending applicationlS'e'r. No."

' 23,756 filed Mar. 23, 1972 for a detailed'descriptionof .such anarrangement, That co-pending applica'tion'is' incorporated herein by reference. t

ln order to firmly hold'the compensatingcoils' in place within their receiving slots, a rigid brace' 'or support structure-110 is interposed between the slot walls and the coilsHThis, bracingstructure preferably.1com= prises a clusterfof strand-like; elements 11 2-which are embedded j the laminations of the slot in the pole piece. In this fash- In the event that the aforementioned rope-like elements of epoxy-coated glass fiber strands are employed as the bracing structure, the pouring step is omitted.

.Rather, after these members have been inserted within the slots, they'are heated so as to cause the epoxy to liquify'. Subsequent curing of the assembly causes the epoxy to become bonded to the coil, the lamination walls, and the strands.

Although a preferred form of the pole shoe has been described in which pairs of inner and outer slots are formed, it should be understood that any number of slots, configured in any suitable manner to receive a "composition of strands and bonding agent, may be provided in accordance with the teachings of this invention.

, It will be apparentthat a pole shoe mounting arrangewithin a hardened binder material i 4- Fro. v

bestos fibers, for example. -Thebinder material preferably comprises a'solventless resin, such asipolyes ter, epoxy,or silicone. The polyestermay be of the type. sold by General Electric Co. under. Designation. No.

702. The silicone mayfbe of the type sold by Dow corning Corp. under Designation No. XR-4'-3l26. When epoxy is used, it is preferably p'rec'oated onto B"-stage 0 glass fiber strands, asare readily commercially 'avaih able in rope-like form,japp roximately inch in diameter, each strand being prec'oated and-semi-curedwith epoxy. These"resinsare typically capable of becoming bonded to the strandsas well as the coil and the'lamin'ations forming the walls of the receiving slots.

With the compensating coils l04 l07 posi tioned within their respective slots 80, 82, 84, 86prior to tightening of the bolts 58, the recesses between .the coils and slot walls may be filled with a cluster of;

strands 112. The clustersmay be braided or twisted intorope-like members, or bound into bundles prior to. insertion. The bolts-58 may be then tightened to tightly ment has been provided in which the windings 15 may be easily replaced, as by loosening the bolts 34, without significantly disrupting 104-107, since the connection between the shoe secthe compensating coils tions 18 and 20 will be maintained.

---MAC'HINE 1ASSEMBLAGE 5O Yoke Assemblage The yoke 14 is preferably assembled by disposing the .platest24 in tight side-by-side abutting relationship, inserting the attachment flanges 30 within the grooves 32, and welding the flanges to the plates.rThis arrangepack the strands between the slot walls and compensating coils. Strands may be added or removed as isnecesmerit avoids the-loss ofspace occuring with known ar- 'rangements in which yoke plates are directly welded together bya filler metal'disposed between adjacent fplates'.)

Pole Shoe 'Intercoupling With the base and tip pole shoe sections 18 and 20 in an individually assembled condition, i.e., with the clampinglbolts 34 tightened and with the fastener assemblies 54 tightened on the connecting bar 50, the compensating coils 104, 105, 106, and 107 are inserted into the associated open slots 80, 82, 84, and 86. Thus, the coils 105 and 106 are slipped into the tip section 20 through the open back side of the slots 82 and 84, while the coils 104 and 107 are inserted between the sepasecond releasable fastener means connected between said connector bar and said base section to secure said tip section to said base section;

said second fastener means being operable independently of said first fastener means to draw said wall means together and clamp said compensator coil windings and said braces together in a manner allowing release of said primary current windings from said base section in the absence of removing said tip section from said base section.

2. Apparatus according to claim 1 wherein said clamping means comprises nuts threadedly received at the ends of said clamping bar; said first and second fastener means comprising a plurality of bolts; and said yoke being recessed around its inner periphery to define pockets in which said base sections are seated.

3. Apparatus according to claim 2 wherein said wall means is arranged such that at least one of said slots of each pole shoe has only one side defined by said base section and at least one other of said slots has at least two sides defined by said base section.

4. A dynamoelectric machine comprising:

an armature;

primary current windings and compensator coil windings;

a yoke disposed around said armature and including:

a plurality of adjacently disposed plates, said plates including aligned recesses that define a plurality of grooves along the outer yoke periphery, and

a plurality of attachment flanges received in said grooves and being bonded to said yoke plates;

a plurality of pole shoes mounted on said yoke, each pole shoe including:

a base section formed of a plurality of adjacently disposed plates, said base plates defining shoulders supporting said primary current windings, and

a tip section formed of a plurality of adjacently dis posed plates, said tip plates defining a curved pole face and including aligned apertures defining at least one opening extending through said tip section remote from said pole face, said tip section further including:

a connector bar. disposed through said opening,

and clamping means mounted at opposite ends of said connector bar for firmly clamping said tip plates together; said base and tip sections being relatively separable and including wall means that cooperatively define a plurality of slots receiving said compensator coil windings;

a cluster of strand-like elements disposed in each slot between said wall means and said compensator coil windings, said elements being bonded together and to said wall means by a binder material so as to define a rigid brace supporting said compensator coil windings against oscillation in said slots;

first releasable fastener means extending between said attachment flange and a respective base section to detachably secure said base section to said yoke;

second releasable fastener means connected between said connector bar and said yoke to secure said tip section to said yoke; said second fastener means being operable independently of said first fastener means to draw said wall means together and clamp said compensator coil windings and said braces together in a manner allowing release of said compensator coil windings from said slots in the absence of removing said base section from said yoke.

5. Apparatus according to claim 4 wherein said clamping means comprises nuts threadedly received at the ends of said clamping bar; said first fastener means comprising a plurality of bolts; said second fastener means comprises a pair of anchor brackets attached to the ends of said connector bar and to said yoke; and said yoke being recessed around its inner periphery to define pockets in which said base sections are seated.

6. Apparatus according to claim 5 wherein said wall means being arranged such that at least one of said slots of each pole shoe has only one side defined by said base section and at least one other of said slots has at least two sides defined by said base section. 

1. A dynamoelectric machine comprising: an armature; primary current windings and compensator coil windings; a yoke disposed around said armature and including: a plurality of adjacently disposed plates, said plates including aligned recesses that define a plurality of grooves along the outer yoke periphery, and a plurality of attachment flanges received in said grooves and being bonded to said yoke plates; a plurality of pole shoes mounted on said yoke, each pole shoe including: a base section formed of a plurality of adjacently disposed plates, said base plates defining shoulders supporting said primary current windings, and a tip section formed of a plurality of adjacently disposed plates, said tip plates defining a curved pole face and including aligned apertures defining at least one opening extending through said tip section remote from said pole face, said tip section further including: a connector bar disposed through said opening, and clamping means mounted at opposite ends of said connector bar for firmly clamping said tip plates together; said base and tip sections being relatively separable and including wall means that cooperatively define a plurality of slots receiving said compensator coil windings; a cluster of strand-like elements disposed in each slot between said wall means and said compensator coil windings, said elements being bonded together and to said wall means by a binder material so as to define a rigid brace supporting said compensator coil windings against oscillation in said slots; first releasable fastener means extending between said attachment flange and a respective base section to detachably secure said base section to said yoke; second releasable fastener means connected between said connector bar and said base section to secure said tip section to said base section; said second fastener means being operable independently of said first fastener means to draw said wall means together and clamp said compensator coil windings and said braces together in a manner allowing release of said primary current windings from said base section in the absence of removing said tip section from said base section.
 2. Apparatus according to claim 1 wherein said clamping means comprises nuts threadedly received at the ends of said clamping bar; said first and second fastener means comprising a plurality of bolts; and said yoke being recessed around its inner periphery to define pockets in which said base sections are seated.
 3. Apparatus according to claim 2 wherein said wall means is arranged such that at least one of said slots of each pole shoe has only one side defined by said base section and at least one other of said slots has at least two sides defined by said base section.
 4. A dynamoelectric machine comprising: an armature; primary current windings and compensator coil windings; a yoke disposed around said armature and including: a plurality of adjacently disposed plates, said plates including aligned recesses that define a plurality of grooves along the outer yoke periphery, and a plurality of attachment flanges received in said grooves and being bonded to said yoke plates; a plurality of pole shoes mounted on said yoke, each pole shoe including: a base section formed of a plurality of adjacently disposed plates, said base plates defining shoulders supporting said primary current windings, and a tip section formed of a plurality of adjacently disposed plates, said tip plates defining a curved pole face and including aligned apertures defining at least one opening extending through said tip section remote from said pole face, said tip section further including: a connector bar disposed through said opening, and clamping means mounted at opposite ends of said connector bar for firmly clamping said tip plates together; said base and tip sections being relatively separable and including wall means that cooperatively define a plurality of slots receiving said compensator coil windings; a cluster of strand-like elements disposed in each slot between said wall means and said compensator coil windings, said elements being bonded together and to said wall means by a binder material so as to define a rigid brace supporting said compensator coil windings against oscillation in said slots; first releasable fastener means extending between said attachment flange and a respective base section to detachably secure said base section to said yoke; second releasable fastener means connected between said connector bar and said yoke to secure said tip section to said yoke; said second fastener means being operable independently of said first fastener means to draw said wall means together and clamp said compensator coil windings and said braces together in a manner allowing release of said compensator coil windings from said slots in the absence of removing said base section from said yoke.
 5. Apparatus according to claim 4 wherein said clamping means comprises nuts threadedly received at the ends of said clamping bar; said first fastener means comprising a plurality of bolts; said second fastener means comprises a pair of anchor bracketS attached to the ends of said connector bar and to said yoke; and said yoke being recessed around its inner periphery to define pockets in which said base sections are seated.
 6. Apparatus according to claim 5 wherein said wall means being arranged such that at least one of said slots of each pole shoe has only one side defined by said base section and at least one other of said slots has at least two sides defined by said base section. 